As a process for removing nitrogen oxides (hereinafter referred to as NOx) from oxygen-rich exhaust gas, ammonia denitration has been put into practical use. However, this process cannot be applied to small combustors because of two reasons: firstly it requires an ammonia source, and secondly release of excess ammonia can cause a secondary environmental pollution problem. It has been found recently, as is disclosed in Japanese Patent Laid Open Sho 63-100919, that NOx can selectively be reduced by hydrocarbons on a zeolite catalyst ion-exchanged with Cu or the like metal.
However, when hydrocarbons as the reducing agent have four or fewer carbons, this catalyst provides low selectivity (molar ratio of hydrocarbons used in NOx reduction to consumed hydrocarbons), and therefore low NOx conversion, in the presence of water vapor which is always contained in general exhaust gases.
Armor et al. reported (in Applied Catalysis B: Environmental, Vol. 1, p. L31) that NOx can selectively be reduced by methane on Co ion-exchanged ZSM-5 (MFI zeolite). However, it has been known that the catalyst is also deactivated to the level insufficient for practical use in the presence of water vapor. These problems of the conventional catalysts urged the search for an improved catalyst which is active even in presence of water vapor.
As a solution to the above-mentioned problems, NOx reduction process using BEA zeolite ion-exchanged with Co (Co-BEA) as a catalyst is proposed in Italian Patent Application No. MI93A2337.
The Co-BEA catalyst is improved substantially in the activity and durability at low temperature in an actual exhaust gas atmosphere containing water vapor etc. However, even this catalyst does not provide high NOx conversion when the exhaust gas temperature is as low as 350.degree. C., when NOx concentration in the exhaust is lower than about 100 ppm, or when the amount of effective hydrocarbons for NOx reduction is quite small in the exhaust. Therefore, a catalyst with higher activity and higher selectivity at low temperature and low NOx concentration has been sought for.
To meet the above-mentioned need, an object of the present invention is to provide a NOx reducing catalyst that is sufficiently active and durable at low temperature and low NOx and hydrocarbons concentrations even in exhaust gas containing water vapor and sulfur oxides (hereinafter referred to as SOx), etc., for use in reducing NOx in exhaust gases, such as those from natural gas combustion, which contain only small quantities of relatively lower hydrocarbons.
Another object of the invention is to provide a NOx reducing process using this catalyst.